Page 45
2. Structural Analysis:
The second phase of the project i.e. structural analysis and modelling was carried out in Abaqus 6.9.The primary focus of analysis was on the wing of the aircraft. The wing was cut in half for ease of calculation and modelling as the behaviour of the left and right wings would be similar.
Modelling Structural Elements
Wing/Skin: The wing was imported from Aeropack into Abaqus as a part; it was then cut along its mid span using the geometry repair function by removing the shell faces.
This was done as only half of the wing was required for analysis and for easy insertion of structural components.
Page 46 I-Beam Flanges: To create I-beam flanges which followed the curvature of the wing, datum lines were made at locations where the spars needed to be inserted.
Partitions in the wing skin were made at these datum line locations and then a copy of the partitioned wing was made, in order to cut the unrequired faces of the skin to form the I-Beam flanges
Page 47 I-Beam/Spars
Once the flanges were obtained, the beams had to be inserted into the flanges to obtain the I-beams which would make up the spars for the wing skin. In order to this the coordinates were obtained from the flanges and then sketched using the Abaqus Sketcher.
2-D side-view sketch of the beams were made and then extruded width wise by 0.025 meters. The beams were then assembled and merged with the flanges to form the I-Beams.
Page 48 Ribs: The ribs were then sketched and extruded the same way as the spars. More co-ordinates were needed for the ribs as they followed the airfoil shape which was more complex than the spars.
Page 49
Page 50 Material Assignment
Material properties now had to be assigned to all the parts, for this purpose materials were created in Abaqus by giving the materials mechanical properties such as Density, Young’s modulus and Poisson’s ratio .
Sections were then assigned to each face of the irregular parts separately, since in Abaqus only sections having the same geometry can be assigned material properties together.
Page 51
Mesh:
After the section assignments were completed, the parts were meshed individually.
The skin was assigned hexagonal elements and the solid parts were given quadratic elements. The spacing between elements was reduced at locations where it was believed that stresses would be higher in order to get an accurate picture while analyzing the assembly. A number of iterations had to be performed till the right mesh for analysis was obtained.
Page 52 Assembly:
After each part was meshed, the ribs were assembled into the spars and then the spar- rib assembly was assembled into the wing. This was done by a series of rotations and translations which took some time to master as these manoeuvres had to be very accurate. The model was then ready for analysis.
Page 53 Analysis:
Vibrational Analysis:
In order to check and verify the behaviour of the model, a vibrational analysis was conducted for the first 10 natural frequencies.
Page 54 Steps
In the ‘steps’ module of Abaqus, the nature of analysis was defined. In this case the frequency step was chosen and the number of Eigen values was entered as 10 for the first 10 natural frequencies.
Constraints:
Since the model was made of different parts, constrains had to be assigned so that the model did not break apart during analysis. The flanges were constrained to the skin of the aircraft and the beams. The ribs were also constrained to the wing skin and spars.
In Abaqus the inner or outer surface selections are determined by 2 colours i.e.
Brown= outer surface and Purple= inner surface.
Page 55
Page 56 Boundary Conditions:
To simulate the wing being attached to fuselage, the wing skin root and the ends of the spars were encastered preventing rotation and translation in all directions at this location.
Interaction Properties:
In order to study debonding interaction properties could be assigned instead of boundary conditions and constraints. However due to the lack of time the interaction properties were not defined.
Page 57 Analysis:
The initial results were very disappointing with large perturbations in the wing skin and in one trial the I-beams broke apart and came out of the skin.
Page 58 The constraints were then adjusted and it was also found that the skin thickness had to be increased. The material properties were also researched again and adjusted as some errors had crept in during conversion from imperial to S.I. units.
The behaviour of the model for the first 10 natural frequencies was then successfully obtained with no perturbations in the skin:
Mode 1:
Page 59 Mode 2:
Mode 3:
Page 60 Mode 4:
Mode 5:
Page 61 Mode 6:
Mode 7:
Page 62 Mode 8:
Mode 9:
Page 63 Mode 10:
Page 64 Linear Elastic Analysis:
A linear elastic analysis of the wing was then carried out at steady level flight during cruise. Since at this condition lift is equal to weight and only half of the wing was being analyzed, the spars were assigned a load of half the weight of the aircraft and the skin was assigned an evenly distributed lift which was equal to half the aircraft weight, in the opposite direction. The weight of the aircraft at cruise was obtained from AAA.
Page 65 The linear elastic analysis job was then submitted yielding the following result. The undeformed shape is shown as shadow under the deformed shape.
Page 66
Conclusions:
Results
A conceptual design of an 8 seater business jet was completed. Vibrational and linear elastic analysis on its carbon fibre composited wing was also done. A procedure has now been established to design an aircraft in AAA and then design and analyze its structural components in Abaqus. Any changes that are required after structural analysis for example change in wing span; root/tip thickness etc. can easily be reinserted into AAA in order to analyze the effect of these changes on performance and if needed the design can be changed and revaluated in Abaqus till an optimum design is achieved.
Page 67
Further Work
This project has great scope for further work. Due to time constraints only vibrational analysis and Linear Elastic analysis in steady level flight of the wing could be completed. If time permitted analysis of delamination could be performed and linear elastic analysis in other flight conditions could be performed on the wings. The fuselage and other components of the aircraft can also be given structural attributes and analyzed. Different materials could be assigned and changes produced could be simultaneously. The aircraft design software AAA was taught as part of the course and therefore there was no need to learn it again for the dissertation. The aircraft design was done in the Howell building and structural analysis was done in the Michael Sterling building of Brunel University. There were some minor delays caused to the project due to upgrades done in the lab, however this was accounted for as a number of copies of the data were made.
Page 68
Gantt chart
Page 69
References:
1) Aircraft Structures for engineering students by T.H.G. Megson 2) Handbook of Adhesives and Sealants by Edward M. Petrie
3) Free online private pilot ground school (http://www.free-online-private-pilot-ground-school.com/aircraft-structure.html )
4) Aircraft design a conceptual approach by Daniel P. Raymer 5) About.com(Composites/Plastics)
(http://composite.about.com/cs/miscellaneousnews/a/bpr_abaqus.htm) 6) Delamination in Composite Materials Dr. Richard Chung San Jose State
University
7) Materials Examination of the Vertical Stabilizer from American Airlines Flight 587 1National Transportation Safety Board, NASA Langley Research Centre, 8) A numerical and experimental investigation of delamination behaviour in the
DCB specimen,Joakim Schöna, Tonny Nyman, 2002
9) Mixed-Mode Decohesion Finite Elements for the simulation of delamination in composite Materials P.P. Camanho ,C.G. Davila
10) Peter Widas ,Virginia Tech Material Science and Engineering
12) Finite Element Analysis, David Roylance , Department of Materials Science and Engineering, M.I.T.
13) Biomesh (www.biomesh.org)
14) European Aeronautic Defence and Space Company (EADS) (http://www.eads.com/800/en/madebyeads/endurance/shm.html)
Page 70 15) Structural Health Monitoring for Life Management of Aircraft, Sridhar
Krishnaswamy , North Western University ,U.S.A 16) CVM, Holger Speckman Airbus, Bremen, Germany 17) Sandia National Laboratories team leader Dennis Roach 18) Structural Health Monitoring ,Fu Ku Chang,2003
19) Composites and Advanced Materials ,U.S. Centennial of Flight
Commission(http://www.centennialofflight.gov/essay/Evolution_of_Technolog y/composites/Tech40.htm)
20) Carbon fiber reinforced plastics in aircraft construction, C. Soutis, 2005 21) Star Telegram (www.star-telegram.com)
22) “Design considerations for composite fuselage structure of commercial
transport aircraft”, G.W. Davis ,I.F. Sakata, NASA Contractor Report CR-159296 23) Online Video Lecture Series on Computational Methods in Design and
Manufacturing by Dr. R. Krishnakumar, Department of Mechanical Engineering, IIT Madras. (http://nptel.iitm.ac.in/)
24) Aeronautics Learning Laboratory for science technology and research(http://www.allstar.fiu.edu/aero/flight12.htm)
25) “Multifunctional self healing and morphing composites”, T. Duenas*1, E.
Bolanos2, E. Murphy2, A. Mal3, F. Wudl2, C. Schaffner2, Y. Wang3, H. T. Hahn3, T. K. Ooi4, A. Jha1,2007,US Army Aviation and Missile Research, Development, and Engineering Centre
Page 71 26) “Intelligent Material Systems Using Epoxy Particles to Repair Micro cracks and Delamination Damage in GFRP”, M. Zako and N. Takano,1999, Department of Manufacturing Science, Osaka University
27) “Self-healing polymer composites”, R S Trask, H R Williams and I P Bond, Department of Aerospace Engineering, University of Bristol
28) “Use of epoxy/multiwalled carbon nanotubes as adhesives to join graphite fibre reinforced polymer composites”, Kuang-Ting Hsiao, Justin Alms,Suresh G Advan,2000
29) UIUC Airfoil Coordinate database(http://www.ae.uiuc.edu/m-selig/ads/coord_database.html)
30) “A carbon strain sensor for structural health monitoring”, Inpil Kang, Mark J Schulz University of Cincinnati ,2006
31) “Intelligent Structural Health Monitoring (SHM) of Composite Aircraft structures using Acoustic Emission sensors”,Dirk Aljets ,2005 32) Net composites (www.netcomposites.com)
33) Azom materials (www.azom.com) 34) Composites and advanced materials
(http://www.centennialofflight.gov/essay/Evolution_of_Technology/composites/Tech 40.htm)
35) “Wing instability of composite wing aircraft” Mahmood ,Fatholla,University of Iran
Page 72 36)
“
Flutter prediction, suppression and control in aircraft compositewings”,Nagarjuna, Cranfield university
Page 73
Appendix :
Guidelines on interchanging between Abaqus and AAA
1) The design should be exported to Aeropack after it is completed in AAA 2) The file format used to export the model from Aeropack to Abaqus should be
IGES
3) While exporting the model from Aerpack care must be taken about the units used as this can effect the whole project. Abaqus uses the same units the user has provided from the beginning and has no predefined units.
4) The whole aircraft can’t be meshed as a whole but can be meshed separately, however this provides little benefit to structural analysis as the model imported from Aeropack is a shell and structural components have to be added to it.
5) Ideally a single component from Aeropack should be imported and given structural attributes.
6) To get coordinates from the wing skin in order to model structures like spars and ribs, the skin or flanges should be given an arbitrary mesh and then the coordinates of the nodes can easily be found using the query option in the tools menu.
Page 74 Email from Grob Aircraft
Dear Mr. Narayan,
All Grob aircraft are produced by use of wet lay-up composite materials.
The resin system for motorplanes is L20/SL (today called ERP L20 / EPH 960).
Some gliders are produced from the Scheuffler resin system L285 / H285, H286, H287. The very old gliders from Epicote / Laromin.
Fibre Fabrics are: Interglas 92110, 92125, 92140, 92145, 92146 and comparable fabrics. Carbon fabrics: Mainly 98141 or ECC 452, also ECC459.
Glas Fibre Rovings: Vetrotex EC9, Carbon HTA Rovings.
We hope that helps.
Best regards Jörg Unbehend
Joerg Unbehend Head of Design
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